A reverse phase high pressure liquid chromatographic technique was developed for measuring uridine levels in serum and tissues. Serum uridine levels were determined in mice, rats and humans and were found to be species-dependent and to fall within a relatively narrow range. Serum uridine and total liver uridine were determine in mice following treatment with inhibitors of the de novo pyrimidine biosynthetic pathway (PALA, pyrazofurin, 6-azauridine) and a stimulator (D-galactosamine) of the pathway. Serum uridine levels were determined in patients receiving maximally tolerated doses of PALA. Studies were initiated to determine the source and regulator of serum uridine. An isolated perfused rat liver system was used in conjunction with a high pressure liquid chromatographic analysis of the perfusate to determine role of the liver in controlling uridine concentrations. Results from this system showed that the isolated rat liver can function to control and maintain uridine concentrations at physiologic levels.